Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 371,370	$ 315,000
Unrealized Gains	12	85
Unrealized (Losses)	(251)	(28)
Estimated Fair Value	371,131us-gaap_AvailableForSaleSecurities	315,057us-gaap_AvailableForSaleSecurities
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	24,915	57,296
Estimated Fair Value	24,915	57,296
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	226,209	182,426
Unrealized Gains	8	62
Unrealized (Losses)	(170)	(16)
Estimated Fair Value	226,047	182,472
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	120,246	75,278
Unrealized Gains	4	23
Unrealized (Losses)	(81)	(12)
Estimated Fair Value	120,169	75,289
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	36,341	113,794
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	251,759	150,892
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 83,030	$ 50,371

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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